The present invention relates to a flaw detector which detects flaws which are sometimes present on the surface of a sheet or on the inner portions of a transparent sheet.
There are known sheet-flaw detection devices which photograph sheets of paper, sheet metal, or vinyl film by means of linear array cameras employing linear image sensors, i.e. CCD line sensors, each of which is composed of several hundred to several thousand photosensitive elements arranged in a linear fashion. From the video signal changes emitted from these sensors, sheet flaws such as pinholes, soiling, wrinkles, and adhering insects can be detected.
However, with these flaw detection devices, completely satisfactory flaw detection has been difficult to achieve, because problems occur in the processing of the signals, due to the different sensitivities of linear arrays, background S/N, and edge blanking. In other words, as the difference in video signals received from a portion with flaws and one without flaws becomes increasingly minute, better quality signal processing is thus required. Accordingly, a new method of flaw detection has become necessary, in order to cope with the aforementioned linear sensitivity variation, background signal S/N, and edge blanking processing problems.